Towers for supporting large structures, such as billboards, wind turbines, fluid containers, communication, power, and other transmission devices, lighting, freeway signs, etc., include support columns that must be firmly secured to the ground to resist overturning forces on the towers. The support columns are secured to the ground by foundations or footings. To resist such overturning forces, foundations must be able to maintain support columns in an upright position despite overturning forces that may act on the columns.
Many conventional tower foundations include a footing embedded within a cavity that is formed in the ground. Typical footings are made mainly of concrete. The support column is secured to the footing by maintaining the column in place within the cavity and pouring the concrete around the column. Over time, the concrete hardens to secure the column to the footing.
Because of the need to resist overturning forces and potential inconsistencies in the ability of the soil near the surface to support vertical and lateral forces, the footing, and thus the cavity, must extend a substantial distance and occupy a substantial amount of space below the surface. For example, some conventional foundations can extend about 30-45 feet below the surface and occupy a space up to about 5,000 cubic feet.
To form a sub-surface cavity large enough to accommodate conventional footings and columns, a substantial amount of earth must be excavated or removed. The larger the excavation, the more labor, materials, and equipment necessary to form the excavation. For example, a crane is required to hold the support column in place while the concrete hardens. As the amount of concrete necessary to form the footing increases, the time it takes for the concrete to harden and the support column to remain in place increases. The longer the support column has to be held in place by the crane, the higher the cost for use and scheduling of the crane. In addition to increased costs for a crane, larger excavation pits result in cost increases associated with auguring and digging equipment for removing earth from the excavation cavity or pit, and water pumping equipment for removing water from pits deeper than the water table. Also, large foundations result in increased costs associated with additional concrete and concrete transportation vehicles.
Relatively large towers often are installed in two stages. First, a footing securing a first portion of the support column is installed in the ground. Second, a remaining second portion of the support column is coupled or spliced to the first portion to form the completed support column. Conventionally, splicing two support column portions together includes bolting a gusseted flange of the first portion to a gusseted flange of the second portion or welding the first portion to the second portion. Each approach requires manually intensive and costly fastening or welding at the fabrication and/or installation site. Further, the two portions of the support column often are out-of-round making splicing difficult.
After installations, structural elements of a tower foundation may fail or tower foundations may no longer be needed in a particular location. Many conventional tower foundations do not allow for easy removal of failed components or the entire tower foundation. Additionally, most conventional tower foundations are not reusable after removal from an installation site. Also, many conventional tower foundations do not allow for post-installation adjustment should a tower be installed incorrectly, such as being vertically misaligned.